I can find nothing on the origin of why this weed is called crabgrass. Any ideas or theories?
I would assume it is because it's crabbed,that is,ill-formed and drawn.
deaconB said
I would assume it is because it's crabbed,that is,ill-formed and drawn.
The Online Etymology Dictionary guesses the same thing as you do. Online Etymology Dictionary: crabgrass
My theory is that if it's green, and you can cut it, it's a lawn, but my wife hates the stuff. For "crab" our old Websters has  "a malignant growth, a cancer." (Obsolete and rare)
Perhaps that's a related meaning? I mean the stuff does take over!
Top of all that is capricious of the human mind must be the association of the poor crab to disease. There is but the vaguest hint at blood vessels on skin inflammations- that's about it ! Any basis in ancient myths even- as disease ? None. But bad mind habits die hard.
Back to crabgrass, if you think 'take over,' wait till you see the lovely asian bamboo.